Digital signal controls sine generator

The circuit of Figure 1 produces an accurate, variable-frequency sine wave for use as a general-purpose reference signal. It includes an eighth-order elliptic, switched-capacitor lowpass filter, IC₃, which uses a 100-kHz square-wave clock signal that microcontroller IC₂ generates. (Any other convenient square-wave source is also acceptable.) The microcontroller receives its clock signal from a 10-MHz oscillator module. A voltage supervisor, IC₁, ensures correct operation in the event of a power failure. IC₃ sets the filter's cutoff frequency at one-hundredth the clock frequency.

The eighth-order elliptic filter's sharp roll-off sharply reduces the harmonic amplitudes in a 1-kHz square-wave input, thereby producing a nearly perfect 1-kHz sine wave at its output. Using divider-chain logic or a processor, you can then create a digitally adjustable sine-wave source by adjusting the clock and input frequencies and maintain a ratio of 100-to-1 between them. To prevent clipping at the positive and negative peaks, attenuate the input signal and superimpose it on a dc level of V_CC/2. The result for a 5V input is a 2.25V peak-to-peak output. Listing 1 shows the assembly code for the microcontroller this application uses.